I tried RCKen's technique - fill an old Windex bottle with washer fluid, add about 6 drops of dish washing detergent (Ivory liquid concentrate in my case since that's what I had on had and use for removing mold release from plastic models). Seems to work as well as any other cleaner I've used.

I've also heard of guys using Simple Green diluted with water, but not having tried it my self I'm not sure how it works. The washer fluid/liquid dish washing soap works good though.

Hogflyer